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VSD vs. VFD (Acronym Confusion)

06/01/2012 2:24 AM

I know that VSD stands for Variable Speed Drive and VFD (Variable Frequency Drive), but most importantly what is the difference between these two short forms. TQ

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#1

Re: VSD vs. VFD (Acronym Confusion)

06/01/2012 3:35 AM

The two terms are largely synonymous.

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#2

Re: VSD vs. VFD (Acronym Confusion)

06/01/2012 5:39 AM

VFD is generally a sub-species of VSD, used on AC synchronous or near-synchronous (squirrel cage) motors to control speed, mainly by changing frequency (voltage-current changes etc are useful byproducts). But generally speaking, you can control speed of eg a DC motor with an appropriate VSD. Electrical All engineers do tend to oversimplify by elimination. That's why rules are enforced in the first place. (lol) S.M.

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#3

Re: VSD vs. VFD (Acronym Confusion)

06/01/2012 1:58 PM

They are NOT synonymous despite the ubiquitous inaccurate use as such. All VFDs are VSDs, but not all VSDs are VFDs.

VSD = Variable Speed Drive

  • VFD (= Variable Frequency Drive) is an electronic VSD for AC Squirrel Cage Induction motors. AFD (Adjustable Frequency Drive) is a full synonym for VFD.
  • LCI (= Load Commutated Inverter) is an electronic VSD for AC Synchronous motors
  • DC Drive is an electronic VSD for DC motors
  • Eddy Current Drive / Clutch is an electrically controlled mechanical VSD used with any kind of prime mover.
  • Vari-Drive is a purely mechanical (belt driven variable pitch pulley) VSD used with any kind of prime mover.
  • CVT (Continuously Variable Transmission) is an electronically controlled mechanical VSD for engine driven systems (used in many new car transmissions now)

I should also note that modern VFDs are also now capable of being variable TORQUE drives, even if speed is kept constant. So in that application, a VFD may NOT necessarily be used as a VSD.

There are other examples. Using "VSD" when discussing variable speed systems is a grossly inaccurate way of speaking and usually is a way of distinguishing the speaker as a non-engineer. We should all strive to be more accurate and set an example.

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#7
In reply to #3

Re: VSD vs. VFD (Acronym Confusion)

06/02/2012 12:26 PM

Hydraulic coupling also can be called a VSD, I guess. This is also called fluid coupling (Varicon for example) and is widely used in power, O&G industries.

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#4

Re: VSD vs. VFD (Acronym Confusion)

06/01/2012 5:41 PM
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#5

Re: VSD vs. VFD (Acronym Confusion)

06/01/2012 10:51 PM

If it is an AC motor, the speed is controlled by the frequency, hence VFD. If you increase the frequency, reactance will increase and the voltage will need to increase as well if you expect to make any power. It is obviously a much more complicated apparatus, especially in multiphase, and I can't imagine anyone would want to employ VFD on a single-phase machine.

If it is a DC motor, speed is controlled by voltage, VSD. That's really simple, and usually done with pulse-width modulation control of MOSFET transistors. This is way easier on the transistors than using the transistors in their linear range which would no doubt burn them up in a few seconds.

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#6

Re: VSD vs. VFD (Acronym Confusion)

06/02/2012 5:10 AM

Dear Mr.Zmun,

VARIABLE SPEED DRIVE and VARIABLE FREQUENCY DRIVE - OPERATIONALLY as the NAME or TOPIC INDICATES- the speed is varied.

THE VSD NEED NOT RELATE to Electrically Controlled speed variation and it is on account of MECHANICAL ASPECT in POWER TRANSMISSION. Example: usage of REEV's Pulley, where the Speed is VARIED by varying the Pitch Circle for the Belt Contact.

VFD necessarily refers to electrical side, by conversion of Power AC to DC and again convert to AC with frequency different from the original Power Supply.

Thanks,

DHAYANANDHAN.S

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#8

Re: VSD vs. VFD (Acronym Confusion)

06/03/2012 8:49 AM

And there's this thing where they take say 220 volts 60 hz AC, take it thru a rectifier bridge, and make it as DC, then they take it again into a inverter and change it in to a AC but now they can control the hertz, and control it from 0 to 400 HZ or even more...

Some reading material. variable frequency drive

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#9

Re: VSD vs. VFD (Acronym Confusion)

06/05/2012 4:10 PM

Sufficient information is given on the subject for the difference between VSD & VFD by above all posts.

I would like to add some more information on the induction motor speed control by Frequency.

Speed control of induction motor by frequency technically is also called as VVVF or V3F control that is Variable Voltage & Variable Frequency control.

Well known Voltage equation for Induction Motor based on induction principle is

V=4.44ABFN----------1(for Induction Motor)

Where: V= Applied Volts; A= Area of magnetic circuit in M2; B=Flux density Web/M2; N= number of turns F= System Frequency.

Equation-1 can be re-written as

V/F = 4.44ABN= KB= constant --------2; (K=4.44AN is constant, B is also maintained constant to avoid magnetic saturation otherwise it draw heavy magnetization current & resulting heavy losses) for a given maximum torque designed. Whenever the load (torque) on induction motor deceases then speed of the Induction motor has to be decreased (power relation with speed for Induction Motor is P1 = P* (n1 /n)3) so as to save power, the speed of the Induction Motor must be decreased by decreasing the frequency (Speed(n) = F/p; p= pair of poles)

From equation-2, it is clear that to change speed of induction motor both voltage (V) and frequency have to be decreased in the ratio of V/F. This is achieved by Pulse Width Modulation(PWM) technique using power Transistors or Thyristors (SCRs) System.

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